The definition for a solvent in the core training manual is " A liquid, such as water, kerosene, or alcohol that a pesticide or other substance will dissolve in and form a solution". That is a valid definition for mosquito control, however there are some unique needs for mosquito control that I will review. I will discuss solvents for larviciding as well as adulticiding.
The most commonly used larvicides are Bacillus sphaericus (VectoLex®), Bacillus thuringiensis israelensis (Aquabac®, Bactimos®, VectoBac®), and temephos (ABATE ®). All of these products have granular formulations that do not need a solvent. They are applied as they come out of the container.
Generally ground applied larvicides are applied in relatively high volumes of water per acre, as much as 40 to 100 gallons per acre. When larvicides are applied by aircraft, they are typically applied at low volume rates of 0.5 to 2.0 gallons of water per acre, however some are applied as ULV applications of less than 0.5 gallons per acre.
One larvicide has a wettable powder formulation and all have concentrate liquid formulations. The wettable powder and liquid formulations are diluted in water prior to application.
Water is widely available and generally inexpensive. There are variations in water however, and one specification that you may want to be aware of is the pH of your water supply. Water that has a high pH ( in excess of pH 7 ) can cause many pesticides to degrade rapidly, and their effectiveness can be reduced by 50% in as short a time as 2 or 3 hours. If you have this situation with you water supply, a buffer can be added to move the pH in to the normal range.
The solvents needed for adulticides are more varied and depend on the type of adulticiding to be performed. The solvents typically referred to on labels and used for adulticiding are No. 2 fuel oil, diesel fuel, heavy aromatic naphtha (HAN), Aromatic 200 Solvent, light mineral oil, refined soybean oil, and water.
Some labels specify the use of light mineral oil as a solvent. There is some confusion about the difference between petroleum oil and mineral oil. All oil begins as crude that comes from the ground. Crude very often is a very thick product that contains many impurities and is not very useful in its natural form.
The crude goes through a refining process that separates various products from it. Refining crude oil is a very complex process that takes years to learn, however for our purposes we can show a flow of products beginning with crude that progress to Aromatic 200 Solvent to diesel to gasoline to mineral oil to National Formulary (NF) to US Pharmacopoeia (USP). The further crude progresses in the refining process, the fewer impurities it contains. Mineral oil is "cleaner" than Aromatic 200, NF products are cleaner still and can be used for cosmetics and animal feeds. USP products are the most highly refined and are used for internal consumption in medications, etc.
Air and ground thermal applications utilize No. 2 fuel oil or diesel fuel. These products comes off relatively early in the refining process, have a relatively high amount of impurities, high unsulfonated residue content (UR) are low in aromatics and have a relatively dark color. They are readily available and generally less expensive than more highly refined oils. They are used primarily with organophosphate adulticides such as FYFANON® and DIBROM.
When organophospates are mixed with No. 2 fuel oil or diesel, a dark thick sludge is typically formed that settles in the bottom of mixing tanks and plugs lines and nozzles in ULV sprayers. To reduce this problem, adjuvants are added to and mixed with the oil before the insecticide is added. Examples are ORTHO ADDITIVE and THIOSPERSE.
Aromatic 200 is used as an additional solvent in some aerial ULV mixtures. It is a good solvent for insecticides because it contains a relatively high proportion of aromatic components. These aromatic components (including naphthalene) have the potential for phytotoxicity, therefor the use of this product should be carefully evaluated. Because the specific gravity of the product is 8.3 LB/gal, it does help aerial ULV sprays settle to the ground better that mixtures with light mineral oil alone. It has a flash point of 203F.
Light mineral oils are more highly refined that the previously mentioned solvents and have the added advantage that they may legally be applied to food crops such as citrus. Because many of the impurities have been removed in the refining process they are not phytotoxic, therefore good solvents for use where there is a lot of vegetation that cannot be damaged.
There are two generally available mineral oils that are used in agriculture and for mosquito control. EXXON USA sells their product ORCHEX® BV Associates sells their product BVA 13. The specifications for BVA 13 are similar to those for ORCHEX®. BVA 13 is EPA registered for use as a solvent for mosquito control products. The product contains 99%+ unsulfonated residue and is non-phytotoxic.
The flash point is 345F. It is odorless and is water white, with a viscosity of 70 SSU@100F. The specific gravity is 7.106 lb./gal. These products are excellent solvents for the pyrethroid insecticides such as permethrin, resmethrin and sumithrin. They produce a good fog for ground ULV applications. Because of their relatively low specific gravity they should be used for aerial applications when an inversion is present so the spray mix will settle to the ground.
Soybean oil is a natural oil that is a good solvent for both pyrethroid
and organophosphate insecticides. It has one major impediment
for mosquito ULV applications in that it leaves a very sticky
film on surfaces from ground ULV applications and hard raised
spots on surfaces from aerial ULV applications.
These residues can be removed with DISHWASHING detergent.
As mentioned earlier water is a solvent that is readily available and can be used with emulsifiable concentrate (EC) adulticides such as CLARKE PERMETHRIN 10EC as a barrier treatment. There are plans for a product that will probably be called AQUAPERMANONE® that will be diluted with water for ULV applications.
Sometimes a solvent is needed to mix two ingredients that are not compatible. Occasionally resmethrin and malathion would be mixed for ULV applications. Since these two products do not mix a co-solvent such as HAN is needed.
Considerations for mosquito ULV applications should include the following. The solvent must be compatible with the insecticide so they will mix well. The solvent should be non-phytotoxic so plants won't be injured with spray applications. The solvent should have no odor or low odor, and should have a low environmental impact. In addition the following considerations should be kept in mind for aerial ULV applications. Because the spray must settle through at least 100 feet to the ground, a solvent with a low evaporation rate and a relatively high specific gravity should be used.
Keeping the above information in mind when choosing solvents should
help you program achieve the mosquito control that you are expected
to deliver.